Preparation of 2-acetyl-1-pyrroline

ABSTRACT

2-acetyl-1-pyrroline is prepared by hydrolyzing a 2-(1-alkoxyethenyl)-1-pyrroline compound with an acid to obtain a reaction medium and then adding an equimolar amount of a base to the reaction medium to obtain a neutralized reaction medium containing 2-acetyl-1-pyrroline.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of application Ser. No.08/158,934, filed Nov. 29, 1993, now U.S. Pat. No. 5,401,521, and which,in turn, is a divisional application of application Ser. No. 07/979,293,filed Nov. 20, 1992, now U.S. Pat. No. 5,280,127.

BACKGROUND OF THE INVENTION

This invention relates to a process for the production of2-acetyl-1-pyrroline.

U.S. Pat. No. 4,522,838 (Buttery, et al.), describes a process for theproduction of 2-acetyl-1-pyrroline. This process comprises catalyticallyreducing 2-acetyl pyrrole for 15 hours under a hydrogen pressure ofapproximately 0.7 bar, to obtain (1-hydroxyethyl)-2-pyrrolidine as anintermediate product, and oxidizing the intermediate product obtainedwith silver carbonate under reflux in benzene, again over a period of 15hours. The 2-acetyl-1-pyrroline is then isolated by gas phasechromatography using a capillary column 2 metres in length and 6.4 mm indiameter. The 2-acetyl-1-pyrroline thus obtained has a purity ofapproximately 95%, but can only be produced in small quantities at atime, due mainly to the purification of the compound by gas phasechromatography. In addition, the 2-acetyl-1-pyrroline is unstable andhas to be stored at a temperature preferably below 0° C., which makes itdifficult to use, particularly on an industrial scale.

Another process for the production of 2-acetyl-1-pyrroline frompyrroline is known from European Patent Application Publication No. 436481, but does not solve the problem in question.

SUMMARY OF THE INVENTION

The present invention provides a process for preparing2-acetyl-1-pyrroline by hydrolyzing a 2-(1-alkoxyethenyl)-1-pyrrolinecompound with an acid to obtain a reaction medium and then adding anequimolar amount of a base to the reaction medium to obtain aneutralized reaction medium containing 2-acetyl-1-pyrroline.

DETAILED DESCRIPTION OF THE INVENTION

In carrying out the present invention, a 2-(1-alkoxyethenyl)-1-pyrrolinecompound, which may be 2-(1-ethoxyethenyl)-1-pyrroline, is hydrolyzedwithan acid, preferably a mineral acid, such as hydrochloric acid, andthen, the reaction medium is neutralized with an equimolar amount of abase, preferably a strong base, such as sodium hydroxide, which may beadded by dropwise addition with stirring. The2-(1-alkoxyethenyl)-1-pyrroline compound to be hydrolyzed preferably isin a concentration of from 2% to 30%, and the reaction medium obtainedby hydrolysis may be diluted with water before addition of the base.

Isolation of the 2-acetyl-1-pyrroline in a powder-form with a supportprovides a stable form which can be stored for a certain time at ambienttemperature. In this case, a support substance of cyclodextrin,maltodextrin, or combinations thereof is combined with the reactionmedium, which provides a support solution, and then, the supportsolution is freeze-dried.

When the support employed is cyclodextrin, it is combined in solution orina dry form with the neutralized solution so that a solution containingup to 20% by weight 2-acetyl-1-pyrroline, based on the weight of thecyclodextrin, is obtained. β-cyclodextrin preferably is employed.

When the support is maltodextrin, it is combined in solution or in a dryform with the neutralized solution so that a solution containing up to10%by weight 2-acetyl-1-pyrroline, based on the weight of themaltodextrin, isobtained. Additionally, gum arabic also may beincorporated into the neutralized solution with the maltodextrin.

The foregoing steps preferably are carried out at a temperature in therange of from -10° C. to 25° C., and it is important to bearin mind thatthe higher the temperature, the greater the risk of decomposition of the2-acetyl-1-pyrroline and the greater the need to workquickly to reducethat risk.

After support addition, the support solution is freeze-dried, and whenthe support is added in dry form, rather than in solution, such mayenable reduction of the amount of water to be removed, which therebyfacilitates the freeze-drying.

Operating in accordance with the present invention, therefore, enablesobtaining a composition of 2-acetyl-1-pyrroline incorporated with asupport in a form of a powder which may contain up to 20% by weight of2-acetyl-1-pyrroline based on the weight of the powder.

Accordingly, the present invention also relates to the use of2-(1-alkoxyethenyl)-1-pyrroline compounds which corresponds to thefollowing general formula: ##STR1##These compounds may be obtained by aprocess in which the corresponding alkyl vinylethyl compound is reactedwith tert-butyl lithium in organic solution, for example in pentane,tetrahydrofuran or ether, or in a mixture of these solvents,N-trimethylsilyl-2-pyrrolidinone is then added and the mixture is leftto react, preferably below 0° C., the solution obtained is hydrolyzed,the organic phase is recovered, dried andpurified to obtain the required2-(1-alkoxyethenyl)-1-pyrroline. The hydrolysis is preferably carriedout by addition of water or a solution ofammonium chloride. Afterhydrolysis, a salt, such as sodium chloride, may be added to saturatethe solution so that the yield of the process can be improved.

The final purification step may be carried out by any method, such asdistillation and/or column chromatography.

Accordingly, compounds of the 2-(1-alkoxyethenyl)-1-pyrroline type canbe obtained by this process and may be used for the production of2-acetyl-1-pyrroline. It has also been found that one of the compoundsof this type, namely 2-(1-ethoxyethenyl)-1-pyrroline, has certainorganoleptic qualities which enable it to be used on its own or incombination as a flavouring agent. These organoleptic characteristicsmay be described as grilled, fruity, hazel nuts and reminiscent of thoseof pyrazine.

EXAMPLES

The invention is illustrated in more detail in the following Examples inwhich parts and percentages are by weight wherein Examples 1-3illustrate preparation and properties of2-(1-ethoxyethenyl)-1-pyrroline, and Examples 4-16 illustratepreparation of 2-acetyl-1-pyrroline and preparation of compositionsincluding that compound incorporated with a support and propertiesthereof.

EXAMPLE 1

A solution containing 54.1 g ethylvinyl ether in 300 ml tetrahydrofuran(THF) is prepared.

This solution is cooled to -40° C. and a 1.4N solution of 429 mltert-butyl lithium in pentane is added dropwise to the solution. Themixture is then stirred continuously for 12 hours at -40° C., afterwhicha solution of 47.1 g of N-trimethylsilyl-2-pyrrolidinone in 300 ml THFis added. The mixture thus obtained is stirred continuously for 7 hoursat -40° C., after which 32.1 g ammonium chloride dissolved in300 mlwater are added. The mixture is then left to return to approximately0°C. and is then saturated by addition of 60 g sodium chloride.Thesolution is then left standing so that the aqueous phase and organicphase can separate. The organic phase is recovered and the aqueous phaseis extracted three times with 50 ml ether.

The various organic fractions obtained are mixed, washed three timeswith 50 ml water saturated with NaCl and then dried over sodium sulfateand thesolvents are evaporated under reduced pressure. 37.8 g crudeextract are obtained in the form of a yellow liquid. A chromatographycolumn 4 cm in diameter and 80 cm in height containing 500 g of a silicagel is prepared.A solution containing 20 parts dichloromethane to 1 partethyl acetate is used as eluent. The crude extract diluted with theeluent is introduced into the column and is then eluted at a rate of 2ml per minute. A 99.9% pure compound in the form of a colourless liquidis obtained in a yield of17.1 g.

EXAMPLE 2 1. Mass Spectrum

The mass spectrum of the compound obtained in accordance with Example 1gives the following results:

    ______________________________________                                               Relative intensity                                                     m/z    (100 = base peak)                                                                             Identification                                         ______________________________________                                        139     8              [M].sup.+  Molecular peak                              124    21              [M--CH.sub.3 ].sup.+                                   110     5              [M--C.sub.2 H.sub.5 ].sup.+                            95     100             [M--CH.sub.3 CHO].sup.+                                94     66              [M--OC.sub.2 H.sub.5 ].sup.+                           83      6              --                                                     82     10              --                                                     67     18              --                                                     41     30              --                                                     ______________________________________                                    

2. Elemental Analysis

Elemental analysis of the compound obtained in accordance with Example 1bycombustion gave the following results:

    ______________________________________                                                         %                                                            ______________________________________                                        C            Observed  68.70                                                               Calculated                                                                              69.03                                                  H            Observed  9.26                                                                Calculated                                                                              9.41                                                   N            Observed  9.74                                                                Calculated                                                                              10.06                                                  ______________________________________                                    

The calculated results are for a compound having the approximate formulaC₈ H₁₃ NO and a molecular weight M of 139.198 g.

3. Infrared Spectrum

The infrared spectrum of the compound obtained in accordance withExample 1in the form of a film gives the following results:

    ______________________________________                                        Frequency of the                                                              characteristic bands (cm.sup.-1)                                                                 % Transmission                                             ______________________________________                                        2977               23.8                                                       2932               28.9                                                       2862               34.5                                                       1738               45.8                                                       1612               26.9                                                       1591               20.5                                                       1370               29.7                                                       1322               19.7                                                       1271               25.1                                                       1129               20.6                                                       1068               25.6                                                        979               34.7                                                        819               30.2                                                       ______________________________________                                    

4. NMR Spectrum

The nuclear magnetic resonance spectrum of the proton of the compound intrichlorodeuteromethane (CDCl₃) at 20° C. shows the followingcharacteristic signals:

    ______________________________________                                                              Coupling                                                Signal   Multiplicity constant                                                (ppm)    of the signal                                                                              (Hz)      Identification                                ______________________________________                                        4.73     Doublet      2.6       H olefinic                                    4.52     Doublet      2.6       H Olefinic                                    4.02     Triplet of a 2.0 and   2H-5                                                   triplet      7.4                                                     3.87     Quadruplet   7         2H Ethyl                                      2.73     Multiplet              2H-3                                          1.93     Multiplet              2H-4                                          1.42     Triplet      7         3H Ethyl                                      ______________________________________                                    

Accordingly, the compound can be identified by these four tests as being2-(1-ethoxyethenyl)-1-pyrroline corresponding to the following formula:##STR2##

EXAMPLE 3 a) Detection of the Perception Threshold by Direct Olfaction

Several aqueous solutions containing 2-(1-ethoxyethenyl)-1-pyrroline invarious concentrations are prepared.

50 ml of each solution accommodated in a 250 ml Erlenmeyer flaskprovided with a cover are presented to six tasters skilled in theanalysis of aromas.

The test is carried out as follows:

Three flasks are presented to each taster: one flask containing thearomatic solution and two flasks containing water. The taster has tosniffthe head space and designate the flask containing the aromaticsolution. The test is repeated twice for concentrations of 1000, 100,10, 5 and 1 ppm.

The following results are obtained:

    ______________________________________                                        Concentration (ppm)                                                                         1000     100    10     5   1                                    ______________________________________                                        Positive olfaction (%)                                                                      100      100    100    50  17                                   ______________________________________                                    

Accordingly, the perception threshold by direct olfaction is of theorder of 5 ppm. b)

Detection of the Perception Threshold by GC Sniffing

The minimum quantity of 2-(1-ethoxyethenyl)-1-pyrroline detectable byolfaction is determined by GC sniffing (a combination of gaschromatography and olfactometry). To this end, solutions of the compoundin various concentrations are analyzed by gas phase chromatography.

After orientation tests, solutions of 2-(1-ethoxyethenyl)-1-pyrroline indichloromethane are prepared in concentrations of 100, 500 and 1000 ppm.

The analyses are carried out under the following conditions:

    ______________________________________                                        Chromatograph HP 5890 A (Hewlett Packard)                                     Capillary column                                                                            DB wax (J and W Scientific),                                                  length 30 m, i.d. 0.25 mm                                       Detection     FID, 250° C. + sniffing port, 150° C.                           effluent splitting: 1/1                                         Injection     200° C., split (27.7 ml/min.)                            Gas vector    helium, 17.5 psi                                                Furnace temperature:                                                                        100° C. - 4° C./min. - 180° C.             Quantity injected:                                                                          1 microlitre of a 1000 ppm solu-                                              tion, i. e., 1 microgram of the com-                                          pound.                                                          Gas flows:    split vent = 27.7 ml/min.                                                     column effluent = 0.6 ml/min.                                                 (sniffing port)                                                 Quantity of 2-(1-ethoxyethenyl)-1-pyrroline arriving at                       the sniffing port = 21.7 ng                                                   Estimation of the air volume entraining the 2-(1                              ethoxyethenyl)-1-pyrroline                                                    Duration of olfactory peak = 5 seconds                                        Humidified air flow rate (make up) = 0.83 ml/sec.                             Air volume estimated at 4.2 ml air.                                           ______________________________________                                    

The perception threshold of 2-(1-ethoxyethenyl)-1-pyrroline is 21.7 ngin 4.2 ml air, i.e., 5.20 ng/ml air.

EXAMPLE 4

2-(1-Ethoxyethenyl)-1-pyrroline is hydrolzed by addition of 5 ml 10.5Nhydrochloric acid to 67.74 mg 2-(1-ethoxyethenyl)-1-pyrroline at 0°C.The mixture is then left standing for 2 hours at ambient temperature(25° C.).

The mixture is then cooled to approximately 5° C. and neutralizedbydropwise addition with continuous stirring of 52.5 ml 1N sodiumhydroxide. An aqueous solution containing 97% 2-acetyl-1-pyrroline,i.e., 52.40 mg, and 3% 2-acetyl-2-pyrroline, i.e., 1.62 mg (compositiondetermined by gas phase chromatography and mass spectrometry) isobtained.

EXAMPLE 5

2-(1-Ethoxyethenyl)-1-pyrroline is hydrolyzed by addition of 5 ml 1Nhydrochloric acid to 69.81 mg 2-(1-ethoxyethenyl)-1-pyrroline at 0°C.The mixture is then left standing for 7 days at ambient temperature (25°C.) so that hydrolysis is complete.

The mixture is then cooled to approximately -5° C. and the hydrolyzedmixture is diluted with 40 ml water and then neutralized by dropwiseaddition with continuous stirring of 5 ml 1N sodium hydroxide.

An aqueous solution containing 54 mg 2-acetyl-1-pyrroline (97%) and 1.67mg2-acetyl-2-pyrroline (3%) is obtained.

EXAMPLE 6

70 ml 1N HCl are added to 1.00132 g 2-(1-ethoxyethenyl)-1-pyrroline(i.e., 7.20 mmol) at 0° C. and the mixture is left standing for 7 daysat ambient temperature.

A first sample A of 17.5 ml of this reaction mixture is diluted in 175ml water and cooled to 0° C. 17.5 ml 1N NaOH and an aqueous solutioncontaining 17.6 g maltodextrin, 2.4 g gum arabic and 430 ml water arethenadded dropwise. The solution thus obtained is freeze-dried in astandard freeze dryer. A white powder containing 2-acetyl-1-pyrroline ina concentration of 1.0%, based on the maltodextrin/gum arabic mixture,and in a concentration of 0.94%, based on the powder, is obtained.

EXAMPLE 7

A sample B of 13.125 ml of the reaction mixture obtained in Example 6 isdiluted in 130 ml water and cooled to 0° C. 13.125 ml 1N NaOH and thenan aqueous solution containing 15 g β-cyclodextrin in 400 ml water arethen added dropwise. The solution thus obtained is freeze-dried as inExample 6. A white powder containing 2-acetyl-1-pyrroline in aconcentration of 1.0%, based on the β-cyclodextrin, and in aconcentration of 0.94%, based on the powder, is thus obtained.

EXAMPLE 8

A sample C of 13.125 ml of the reaction mixture obtained in Example 6 isdiluted in 100 ml water and then cooled to 0° C. 15.0 g β-cyclodextrinin powder form and 13.125 ml 1N NaOH are then added. After stirring for30 minutes at 0° C., the solution is freeze-dried as in Example 6. Inthis case, freeze-drying is easier to carry out because the volume ofwater is much smaller by comparison with Examples 6 and 7.

A white powder containing 2-acetyl-1-pyrroline in a concentration of1.0%, based on the β-cyclodextrin, and 0.94%, based on the powder, isobtained.

EXAMPLE 9

A sample D of 13.125 ml of the reaction mixture obtained in Example 6 isdiluted in 130 ml water and cooled to 0° C. 13.125 ml 1N NaOH and anaqueous solution containing 1.5 g β-cyclodextrin in 40 ml water are thenadded dropwise. The solution thus obtained is freeze-dried as in Example6. A white powder containing 2-acetyl-1-pyrroline in a concentration of10.0%, based on the β-cyclodextrin, is obtained.

EXAMPLE 10

The stability of the 2-acetyl-1-pyrroline prepared in accordance withExamples 6 to 9 is studied over a period of 110 days at various storagetemperatures. To this end, samples of 100 mg freeze-dried powder aretakenafter storage and dissolved in 1 ml water at 0° C. 1 ml ethylacetate containing 1 mg trimethyl-2,4,6-pyridine is added and themixture is stirred for 30 seconds. The mixture is then centrifuged for15 minutes at -5° C. and the organic phase is recovered and analyzed bygas phase chromatography.

The percentage of 2-acetyl-1-pyrroline being decomposed after 110 daysstorage is then determined. The following results are obtained:

    ______________________________________                                        Support and % decomposition of 2-acetyl-1-pyrroline                           Storage  Maltodextrin                                                                             β-Cyclodex-                                                                         β-Cyclodex-                               temperature                                                                            1%         trin 1%    trin 10%                                       ______________________________________                                         20° C.                                                                         Complete de-                                                                             99%        91% After 13 days                                       composition                                                                   after 50 d                                                             4° C.                                                                         33%        10%        13% After 23 days                              -20° C.                                                                         13%         0%        --                                             ______________________________________                                    

It can be seen that 2-acetyl-1-pyrroline in a concentration of 1% onβ-cyclodextrin remains stable for at least 110 days when stored at a lowtemperature. This is also the case, although to a lesser extent, if the2-acetyl-1-pyrroline is in a concentration of 1% on maltodextrin/gumarabic. By contrast, the stability of the 2-acetyl-1-pyrroline decreaseswhen its concentration on the support (in the present caseβ-cyclodextrin) increases. By comparison, 95% pure 2-acetyl-1pyrrolineprepared in accordance with the prior art degrades rapidly in storage at-20° C. (Buttery, et al., Journal of Agric. Food Chem. (1983), 31,823-826).

EXAMPLE 11

1 ppm 2-acetyl-1-pyrroline in a concentration of 1% on β-cyclodextrin isadded to a corn soup just before consumption. The soup thus prepared anda control soup containing no 2-acetyl-1-pyrroline are presented to agroup of six trained tasters. The 2-acetyl-1-pyrroline contributestowardsa rounder, more cooked and more pleasant perception of the food.The "cooked cereal", "popcorn" and "very slightly grilled" notes arestrengthened and developed together with a very fine "buttery-fresh"note.

EXAMPLE 12

1 ppm 2-acetyl-1-pyrroline in a concentration of 1% on β-cyclodextrin isadded to a chicken soup just before its consumption. The soup thusprepared and a control soup containing no 2-acetyl-1-pyrroline arepresented to a group of six trained tasters. The 2-acetyl-1-pyrrolinecontributes towards reducing the "chicken fat" note and strengthens the"chicken meat" and "slightly grilled" notes. The whole is more cookedand more complete and the slight "grilled meat" aftertaste is prolonged.

EXAMPLE 13

1 ppm 2-acetyl-1-pyrroline in a concentration of 1% on β-cyclodextrin isadded to a beef soup just before consumption. The soup thus prepared anda control soup containing no 2-acetyl-1-pyrroline is presented to agroup of six trained tasters. The 2-acetyl-1-pyrroline contributestowardsstrengthening the "slightly grilled meat" note. The impression inthe mouthis more round and the beef aftertaste is prolonged.

EXAMPLE 14

A composition of the "breadcrust" type is prepared by adding thefollowing compounds to 1 liter ethanol: 50 g 2-acetyl pyrazine, 10 g2-acetyl thiazole, 30 g diacetyl, 5 g 2-ethyl-3-methyl pyrazine. 0.1 gof this composition is added to 1 liter water previously salted with 3 gNaCl per liter. The aqueous mixture is divided into two batches. 0.5 ppm2-acetyl-1-pyrroline in a concentration of 1% on β-cyclodextrin is addedto the first batch. The second batch serves as control. A panel of tenpeople compares the two batches. The first batch appears better than thesecond with a strengthened "cereal", "breadcrust" note and a rounded"grilled" note. The whole remains longer in the mouth. When added to apizza dough, the present composition strengthens the "breadcrust" note,above all on olfaction.

EXAMPLE 15

A composition of the "corn" type is prepared by adding the followingcompounds to 1 liter ethanol: 5 g 2-acetyl pyrazine, 5 g 2-acetylthiazole, 0.5 g diacetyl, 20 g dimethyl sulfide. 0.1 g of thiscompositionis added to 1 liter water salted beforehand with 3 g NaCl perliter the aqueous mixture is divided into two batches. 0.5 ppm2-acetyl-1-pyrroline in a concentration of 1% on β-cyclodextrin is addedto the first batch. The second batch serves as control. A panel of 10people compares the two batches. The first batch has a more marked, morecomplete and morepowerful "sweet corn" and "popcorn" note. Persistencein the mouth is more pronounced.

EXAMPLE 16

A composition of the "potato" type is prepared by adding the followingcompounds to 1 liter ethanol: 5 g 2-acetyl thiazole, 5 g trimethylpyrazine, 0.5 g diacetyl, 2 g 2-ethyl-3-methoxypyrazine, 50 gmethylthio-3-propanal. 0.1 g of this composition is added to 1 literwatersalted beforehand with 3 g NaCl per liter. The aqueous mixture isdivided into two batches. 0.5 ppm 2-acetyl-1-pyrroline in aconcentration of 1% on β-cyclodextrin is added to the first batch. Thesecond batch serves as control. A panel of 10 people compares the twobatches. The first batchappears better than the second and has a morecomplete and strengthened note of the "cooked potato flesh" type.

We claim:
 1. A process for preparing 2-acetyl-1-pyrroline comprisinghydrolyzing a 2-(1-alkoxyethenyl)-1-pyrroline compound with an acid toobtain a reaction medium and then adding an equimolar amount of a baseto the reaction medium to obtain a neutralized reaction mediumcontaining 2-acetyl-1-pyrroline.
 2. A process according to claim 1wherein the 2-(1-alkoxyethenyl)-1-pyrroline compound is2-(1-ethoxyethenyl)-1-pyrroline.
 3. A process according to claim 1 or 2wherein the process is carried out at a temperature of from -10° C. to25° C.
 4. A process according to claim 1 or 2 wherein the base is addeddropwise to the reaction medium.
 5. A process according to claim 1 or 2further comprising diluting the reaction medium with water prior toadding the base.
 6. A process according to claim 1 or 2 wherein the acidand 2-(1-alkoxyethenyl)-1-pyrroline are added together for hydrolysis sothat the 2-(1-alkoxyethenyl)-1-pyrroline is in a concentration of from2% to 30% by weight.
 7. A process according to claim 1 or 2 wherein theacid is a mineral acid and wherein the base is a strong base.
 8. Aprocess according to claim 1 or 2 wherein the acid is hydrochloric acid.9. A process according to claim 1 or 2 wherein the base is sodiumhydroxide.
 10. A process according to claim 1 or 2 wherein the acid ishydrochloric acid and wherein the base is sodium hydroxide.